Method of preparing silicon



Aug. 25, 1959 H. c. THEUERER 2,901,325

- METHOD OF PREPARING SILICON Filed July 22, '1955 4 Sheets-$heet 2 FIG.4

SILICON 438 RESERVOIR HYDROGEN AND WATER VAPOR EXHAUST AUXILIARY INLETWATER INVENTOR H. C. THE UERER A TTORIVEY 1959 H. c. QIHEUERER 2,901,325

METHOD OF PREPARING SILICON Filed July 22, 1955 r 4 Sheets-Sheet 3 FIG.5

I 502 1 V f i I 1 56 55 1 52 ,1, I I i I I. 1- l l 53 u v I 503 INVENTORH. C. THEUERER Y W 6Q ATTORN'E V RESIST/VI TY OHM- CM H. c. THEUERER2,901,325

METHOD OF PREPARING SILICON Aug. 25, 1959 Filed July 22, 1955 4Sheets-Sheet 4 FIGS RES/ST/V/TY OHM- CM LOCATION ALONG ROD IN INCHESFIG. 7

LOCATION ALONG ROD IN INCHES INVEIVTDR H. C. THE UERER A TTO'RNEY UnitedStates Patent METHOD or PREPARING SILICON Henry C. Theuerer, New York,N.Y., assignor to Bell Telephone Laboratories, Incorporated, New York,N.Y., a corporation of New York Application July 22, 1955, Serial No.523,897

Claims. (Cl. 23-2235) This invention relates to an improvement inmethods for preparing pure silicon, and relates in particular totechniques for the removal of boron impurities from silicon. Theapplication is filed as a continuation-in-part of the copending patentapplication of J. H. Scaff and H. C. Theuerer, Serial No. 236,662, filedJuly 13, 1951, now Patent No. 2,753,281.

The conductivity properties of elemental silicon are largely determinedby trace concentrations of foreign materials present in the silicon.Close control of the kind and quantity of such impurities is necessaryif good electrical characteristics and reproducibility of properties insuccessive silicon samples are to be obtained.

The silicon intended for use in semiconductor devices such astransistors, rectifiers, lightning arrestors, or photocells, forexample, is generally required to be of a uniform, high-purity grade notordinarily available from commercial sources. The degree of puritydesired in silicon used for such semiconductor applications is commonlybeyond the scope of conventional purification methods heretofore used inthe chemical art. The invention described herein is pertinent to theproduction of extremely pure silicon particularly suitable for use inelectrical devices of the kind mentioned.

Relatively pure silicon may be obtained either by initial formation fromuncontaminated silicon compounds, or may be the product of refiningprocesses working on the less pure elemental metal itself. Whereeffective techniques for the refinement of the starting materials exist,the synthesis of silicon of high purity is feasible. Thus, for example,the silicon obtained by a careful reduction of silicon tetrachloridewith zinc or hydrogen has a purity commensurate with the care exercisedto obtain reactants free of foreign substances.

The cost of such methods of silicon production is considerable ifextensive refinement of starting materials is required. Much attentionhas been paid to processes for refining commercial grades of siliconproduced by less painstaking methods. Commercial grade silicon, ofapproximately 96 percent purity, is prepared, generally, by carbonreduction of silicon dioxide in an electric arc. The impurities inherentin such material are predominantly iron, aluminum, boron and phosphorus.Some of these impurities may form a second nonsilicon phase in the bulkmaterial, depositing at the grain boundaries of the metal. As taught inUnited States Patent 1,386,227, issued on August 2, 1921, to FrederickMark Becket, crushing the material to fine particles and leaching inhydrofluoric acid may dissolve much of the second phase, often raisingthe purity of the remaining silicon to above 99 percent.

For further removal of contaminants, physical techniques have provedmore effective than additional chemical processing of the siliconresidue. The principles of two such physical methods, normal freezingand zone melting, are described by W. G. Pfann in the paper entitledPrinciples of Zone Melting, published in Transactions of the AmericanInstitute of Mining and Metal- 2,901,325 Patented Aug. 25, 1959 icelurgical Engineers, volume 194, pp. 747-753, in 1952. Both of themethods utilize the differential solubility of a solute, such as acontaminating impurity, in liquid and solid solvent, such as molten andsolid silicon, to isolate the solute in one or the other of the twosolvent phases in equilibrium. The efficiency of the processes asdescribed in the publication mentioned, is dependent in part on thedistribution coefiicient, k, of the solute or contaminant. Thecoefficient k is defined as the ratio of the concentration of the solutein the solid solvent to its concentration in the liquid solvent atequilibrium.

The greater the departure of k for a solute from a numerical value of1.0, the greater is the tendency for the solute to isolate itself ineither the liquid or solid solvent phase, leaving the other phasepresent poorer in solute content. With the aid of such purificationsteps, the concentrations of Al, Fe, and P, which are impurities havingk values much diiferent from unity, can be greatly reduced from theirvalues in commercially available silicon. The k value for phosphorus isabout 0.3, for example; that of aluminum lies between 0.0015 and 0.0036. By selective fusion and solidification, these con taminating solutesmay be effectively isolated in chosen portions of a silicon body. Theremainder of the body, which has been thus purified, is the desiredportion. The silicon with a high concentration of impurities may bediscarded.

As the distribution coefficient of a contaminating solute approachesunity, however, the tendency of the contaminating solute to distributeitself preferentially decreases, rendering purification by a methodbased on such a distribution less efiicient. Boron, with a distributioncoefiicient of about 0.8, is a silicon impurity relatively difficult toisolate using either normal freezing or zone refining techniques. Thepresent invention, which comprises the exposure of molten silicon toatmospheres in which water vapor is present as a sole or componentingredient, effects the conversion of boron impurities to volatilecompounds removable from the liquid silicon. The use of an atmospherecontaining water vapor as a means for converting boron contaminants insilicon to substances which can be driven from the silicon with ease isparticularly efiective when used jointly with normal freezing or zonemelting. These latter processes depend also on the production of aliquid phase of silicon, and an economy of time and effort may beeffected by the practice of either of the refining steps in thebeneficial atmospheres described herein. However, the contact of suchatmospheres with liquid silicon, following the specification set outbelow, will bring about the removal of boron as a contaminant if doneindependently, as Well as if done in conjunction with or in addition toother prior, subsequent, or concurrent refining processes.

In the accompanying drawings:

Fig. l is a front elevation, partly in section, of an apparatus foundparticularly effective for the purification of silicon simultaneouslyusing a floating zone refining technique and the Water vapor treatmentdescribed herein;

Pig. 2 is a perspective view, partly in section, of a deviceadvantageously used with the apparatus of Fig. 1 to concentrate or focushigh frequency electromagnetic .waves useful for induction heating intoa planar eonment of silicon with water vapor, in which apparatus arefinement is also accomplished by subsequent normal freezing;

Fig. 5 is a front View, partly in section, of a device for altering thelength of a vertically suspended semiconductor ingot to compensate foranomalies in the diameter of theingot, said anomalies arising fromvolume changes in. the, semiconductor material upon fusion orsolidification;

Fig. 6 presents plots of the resistivity of a silicon rod as; a functionof distance from one end along the rod, each curve being descriptive ofthe resistance characteristics ofthe same rod measured at various stagesof treatment using a preferred example of the water vapor purificationdescribed in this application; and

Fig, 7 presents plots of the resistivity of a silicon rod as a, functionof distance along the rod, successive portions. of the curves plottedbeing descriptive of the resistance characteristics of the rod afterexposure of different segments of the rod to atmospheres containingdifiering amounts. of water vapor.

In Fig, 1 is shown a tubular body 11, conveniently having quartz walls,and being one inch in approximate diameter andaboutZOinches in length.At the ends of the tube 11, caps 12 and 112, conveniently made of brass,are fitted. Each cap 12 and 112 is wrapped with watercar rying coolingcoils 13, and is fitted with a side arm 14 which may serve as an inletor exhaust for gas being passed through the tube 11. Within each end cap12 and112 is mounted a chuck 18, the chuck in the upper cap 112beingfurther equipped with means, including a threaded, screw 111, whichpermit raising or lowering of the upper chuck along the axis of the tube11. Grasped in the chucks 18 are support rods 15 of a refractorymaterial such assilica, each being conveniently about six inches inlength. The support rods 15 terminate in hollow cylindrical cups 17which are advantageously fashioned of the same material as constitutesthe support rods 15 and which are usually made integral with said rods15. The hollow cups 17 receive a silicon rod 16, conveniently held inplace in the cups 17 with a refractory cement such as a mixture ofsilicon dioxide and sodium silicate.

Concentrically surrounding the tube 11 is an induction heating coil 19,which, when carrying high frequency current, serves to heat and liquefythat portion of the silicon rod 16 suspended within the plane of thecircumferential coil.

In the operation of the above-described device, the principles of whichare more fully explained in the copending application of H. C. Theuerer,Serial No. 326,561, filed December 17, 1952, a molten zone isestablished in the vertical rod 16. Surface tension forces serve to keepthe rod integral, even though a portion of the rod is liquefied. Zonerefining can thus be accomplished without confinement of the silicon invessels which might be a source of contamination.

Before initiation of the zone melting process, the silicon rod 16 isusually composed of two separate se ments. The lower portion is a lengthof previously preparedsingle crystal silicon held in the lower end cap12. In the corresponding assembly at the upper end of the tube 11, thesilicon to be purified is attached. By vertical movement of the upperchuck 18, using the screw 111 in theupper end cap 112, the upper portionof the silicon rod 16 can be moved relative to the lower portion astopforrn a gap of desired size between the two segments of the rod.

When the zone melting process is begun, a torch or resistance heater isused to preheat rod 16 where the two segments abut, Induction heating isthen applied, and when a molten region is formed, the rod segments arejoined with the molten zone between them. By means ofga mechanism 113,comprising a motor, gear box, and vertically movable platform, theentire apparatus may be lowered through the plane of the fixed inductioncoil so, that the liquid zone initiated at the joinder of the seedcrystal and the sample to be purified proceeds upward through thesample. The length of the support rods 15 and the passage of waterthrough the cooling coils 13 prevent heat from the molten zone fromaifecting wax seals which may be used in part to fasten the caps 12 and112 to the tube 11. Recrystallization, first taking place at the seedcrystal-sample junction, brings about the formation of a single siliconcrystal built up from the seed crystal, though the original siliconsample may be initially polycrystalline.

The silicon rods to be purified in the apparatus may be obtained by asintering of powdered commercial silicon. The process is taught by R.Emeis in Zeitschrift fiir Natiirforschung, volume 9A, book 1, publishedin 1954, at page 57. Conveniently, the rods may be produced also by ahydrogen reduction of silicon tetrachloride with deposition of thesilicon on a filament. This process is described in an article by RudolfHolbling in Zeitschrift fiir angewandte Chemie, volume 40, published in1927, at page 655. The use of a purification step subsequent to theformation of the silicon material permits the use of reactants whichhave not been extensively purified prior to reaction. However, thegreater the attention given to purity before performing the reductionreaction, the purer the resultant product. Less prolonged use of thewater vapor treatment may be required to remove impurities, inconsequence.

In a preferred application of the apparatus of Fig. 1 to water vaporpurification of silicon, hydrogen conveniently saturated with watervapor at 0 C. was admitted into the tube 11 through the inlet 14 in theupper end cap 112. The gas was removed through the corresponding outlet14 in the lower cap 12. A flow of about 1 liter per minute of gas wasmaintained. The lowering mechanism 112 was so adjusted to give amovement of the silicon sample 16 through the induction heater 1? at aconvenient rate of one-tenth inch per minute.

Fig. 2 shows a more detailed view of a coil, which may be used as thecoil 19 of Pig. 1, useful for heating metallic materials by induction inapparatus like that shown in Fig. 1. The coil comprises a hollowcircularlybent tube 21, conveniently of copper tubing one-fourth inch inoutside diameter, with a wall thickness of about one-sixteenth inch.Soldered along the inner circumference of the circularly bent tubing 21,is a fin 22, conveniently also of copper. The fin 22, whose outsidediameter matches the inside diameter of the circle formed by the tubing21, has a cross section, as shown, which is roughly T-shaped. One arm ofthe T furnishes a surface solderable to the tubing 21, While the otherperpendicular arm, extending radially inward from the curved tubing 21,tends to define a radial plane within the circle formedby the tubing 21.

In practice, the ring of tubing 21, with its attached fin 22, ispositionedso that it circumferentially surrounds the metallic body to beheated. When the tube 21- is connected with a source of high frequencycurrent, the fin 22. acts to concentrate the radiation passing throughthe conductor 21 in such a manner that only a thin ring of the metalsuspended within the plane of the circularly bent tubing 21 and fin 22is heated. Such a focussing of the radiation permits more intenseheating of a portion of the metal being so heated. When used inapparatus such as that shown in Fig, 1, use of the fin also helps toprevent the formation of a molten zone so large as to disrupt thesurfacetension forces maintaining an integral column of semiconductormaterial. Such tapered coils are particularly useful when small diameterrods are being refined, as they produce a molten zone of smalldimensions, in such rods. With rods of diameters comparable to A inch ormore, focussing coils are usually unnecessary.

For inductive heating of the rod processed in the floating zoneapparatus shown in Fig. 1, a current with a frequency of 5 megacyclesper second. is mostadvantageously used. High frequencies of this.magnitude. give,

5 good heating of polycrystalline material, as well as of single crystalsilicon. Further, less stirring and agitation of the molten zone isobserved than when presently available generators of lower frequenciesare used.

In Fig. 3 is shown a more conventional zone refining apparatus modifiedto permit simultaneous water-vapor purification. In the figure,induction heating coils 31, conveniently having an inside diameter ofone and seveneighths inches, are wound around a tube 32 made of arefractory material such as quartz, and conveniently being about one andone-half inches in outside diameter. The coils in the specificembodiment pictured are clustered along the tube 32 so that each seriesof coils will heat a section conveniently one and one-fourth incheslong. The lateral spacing between the centers of successive coilclusters is about three and one-half inches for the apparatus shown. Thetube 32 is constricted and fitted with a stopcock 33 at one end, andfitted with a stopper 34, conveniently made of rubber, at the other. Ahollow tube 303, conveniently made of silica, penetrates the stopper 34.The tube 303, about one-fourth inch in outside diameter and tapered toabout one-sixteenth inch at one end, is positioned to play a stream ofgas over the surface of a boat 301 resting within tube 32.

The tube 32 is clamped securely by circular clamps 35, which are in turnattached to a horizontally movable platform 36 which rests on a track37. The platform 36 may be moved along the track 37 in one direction bymeans 38 comprising a motor and gear box, and returned to the startingposition at the other end of the track 37 by the action of an extendablespring 39. A tripping mechanism, not shown, permits the spring 39 topull back the platform free of the action of the mechanism 38, and thenresets said mechanism 38 to initiate another cycle of platform movementagainst spring tension.

The passage of a molten zone through a horizontal silicon ingot duringzone refining in a crucible tends to taper the ingot by accumulation ofthe silicon being refined in those portions of the ingot last liquefied.This phenomenon, known as matter transport, has been discussed andanalyzed in a paper by W. G. Pfann entitled Change in Ingot Shape DuringZone Refining, published in the Journal of Metals, volume 5, pp.1441-1442, in November 195 3. As there taught, such tendency for mattertransport can be reduced by inclining the silicon ingot upward in thedirection of zone movement. In the apparatus shown in Fig. 2, zonemovement is from left to right, as drawn, and .the right end of theapparatus is raised by inclining the track 37 from the horizontal. Theentire apparatus, including the crucible 301 containing the ingot, isthen similarly inclined.

A convenient angle of inclination, 0, is obtained by raising the track37 three inches out of the horizontal at the right end. As the track 37in the apparatus shown has a length of 27 inches, the angle ofinclination, 0, is defined as slightly greater than 6. The best valuefor such an angle of inclination is usually most easily found byexperiment, as the value is dependent on the specific design of therefining apparatus and on the material being refined.

Within the tube 32, which in the pictured example has a convenientover-all length of about forty-two inches, is the semicylindrical silicaboat 301, earlier mentioned. For a tube 32 of the length given, asilicon boat having the following dimensions has been used to advantage:width-one inch; heightthree-fourths inch; length-fifteen inches;thickness of wall-one-sixteenth inch.

In operation, the boat 301 is filled with powdered silicon to berefined. Because of the bulky volume of the powdered material, it isfound convenient to load the boat with more dense slugs of silicon,formed by compression of the powder. A small piece of lump silicon isadded to the boat, and the whole inserted into the tube 32 after removalof the stopper 34. The boat or crucible 301 is advantageously set on asilica paddle 302, and the boat and paddle laid on the floor of the tube32. Use of a paddle 302 facilitates loading and unloading. The paddle,further, serves as an insulating or heat-dissipating layer, tending toprevent a possible tendency for the tube 32 to soften because of thedirect transmission of heat thereto from the boat 301 containing moltensilicon. Interposition of the silica paddle 302 aids in dissipating heatfrom the crucible 301, thus reducing the chance of causing cohesionbetween the boat 301 and tube 32 which might arise if the two were indirect contact.

After loading, the stopper 34 is reset into the tube 32. A stream ofhydrogen is passed through the tube 32 from the inlet tube 303 to theexhaust 33, and a high frequency current is passed through the coils 31.Inductive heating of loose, powdered materials is inefficient, as isheating of even the relatively more dense compressed slugs, so the highdensity silicon lump earlier mentioned, which is more subject to beinginductively heated, is put in the boat 301 and so positioned as to beheated by one of the coils. Initial heating of the lump with aresistance heater may be used to speed fusion. Once liquefied, thenow-molten material conducts sufficient heat to melt adjacent portionsof the silicon powder and induction heating of the resultant, moredense, liquid metal is easily brought about.

The platform 36 is initially set, for convenience, at that one of thetwo extreme positions of the travel cycle for which the spring 39 isunder least tension.

With an apparatus of the type pictured in Fig. 3, four molten zones, andoften a portion of a fifth, can be created in the silicon contained inthe boat 301. The mechanism 38 is started and the platform 36, with thetube 32 attached thereto by the rigid supports 35, is drawn through thefixed coils 31 at a convenient rate of one-fourth inch per minute. Thesilica boat, filled with silicon, is thus passed through the coils, amolten zone being formed at several positions in the silicon, withmaterial liquefying or crystallizing as it passes into or out of thespace within the induction coils.

A three and one-half inch horizontal displacement of the tube 32 and theboat 301 relative to the coils 31 is usually effected before thetripping mechanism permits the spring 39 to draw back the platform 36along the track 37 and resets the mechanism 38 for the start of anotherrefining cycle. In this way, a molten zone formed under one of the coilsis carried three and onehalf inches from left to right along the lengthof the ingot during a refining cycle. When the spring mechanism istripped, that molten zone is now newly stationed under the next coil tothe right of the coil under which the zone started during the previouscycle. The process is repeated with new molten zones being formed at theleft end of the crucible 301, moving through the ingot, and disappearingat the right end of the ingot. If, as is found useful in the operationof the specific apparatus shown in Fig. 3, the cycle is repeated about20 times, the effect is that which would be achieved by passing theentire length of the bar through a single coil 20 times, bringing aboutsome purification on each pass through the coil.

During the zone refining process, a hydrogen and water vapor mixture isadmitted into the tube 32 through the hollow tube 303. The tube 303 isconstricted at that end inside the chamber 32. It may also be bentdownward slightly in the direction of the boat 301. These modificationsof tube 303 are to promote a streaming of the hydrogen-water vapormixture over the surface of the silicon melt in the boat 301. Suchstreaming tends to prevent localized depletion of water vapor in thepurifying amosphere. Uniform purification of the ingot being refined isthus enhanced.

Another design, not shown, of a tube similar in function to tube 303,extends the length of the boat 301 and is perforated in several placesto permit a play of the 7 refining atmosphere on the boat along itsentirelength.

The exhaust gas leaves the tube through the stopcock 33. Again, a rateof flow of gas of one'liter' per minute has been found convenient. Thehydrogen is pre= viously saturated with water vapor at a temperature of,conveniently, 10 C. Removal of boron according to the principles of thenew technique goeson concurrently with purification accomplished bymultiple zone refining passes.

In Fig. 4 is shown an apparatus which has proved particularly useful inthe application of water vapor purification to silicon, coupled withpurification brought about by normal freezing. The drawing shows a tube41. of a refractory material such as silica, the lower portion of whichis concentrically surrounded byinduction heating coils 42,. The coilsare mounted on a platform 43 which is capable of vertical movement withrespect to the fixed tube 41. Means 44, comprising a gear box and motor,are provided to control the movement of the platform. Within the tube 41is a heat shield 46, preferably of a refractory material such asAlundum, serving to minimize heat loss by transfer from a graphiteheater crucible 47 to more radially outward areas. The graphite crucible47 is that part of the apparatus directly heated by operation of theinduction coils 42, and holds a thin-walled silica crucible 48. in whicha silicon melt 4-9 in turn is held. All the parts, the shield 46 andnested crucibles 47 and 43, rest on a bed ofrefractory material 45,conveniently sand, at the bottom of the tube 41.

At its upper extremity, the tube 41 is sealed with a suitable cementinto a metal furnace head 431, which latter'is wrapped with coolingcoils 432 and is equipped with an inlet 433 for the admission of gasesinto the apparatus. The top of the fumace head 431 is sealed with acover 434 bolted to the furnace head. A tight seal is made with gaskets,not shown, of lead or other suitablematerial. Extending through thecover'434 and capable of extension into thesilicon melt 49 is a silicatube 435, equipped with a silica tamp 436. The tube 435is used forintroduction of powdered silicon from a reservoir 438 into the melt 49.A silica funnel 439 aids in directing the powdered material into thecrucible 48. A second silica tube 4-37 is used to bubble hydrogen andWater vapor mixtures through the molten. silicon 49'. Both tubes 435 and437 are fitted into the cover 434 by means of flexible rubber sleeves401. These sleeves 401 are fastened at their upper end to stoppers 402of rubbet, or other suitable material, which tightly surround the tubespassing through them. At their lower end, the

flexible sleeves 401 are cemented or wired to metal tubes, not shown,which project through the cover 434. In this manner, a gas-tight seal ismaintained which still permits vertical and some lateral movement of thesilica tubes 435 and 437.

Finally, the cover 434 is equipped with a viewing port 4% for visualinspection of the melt 49 or determination of the melt temperature byoptical means. There is also an exhaust outlet 4% for the escape orwithdrawal of vapors from the apparatus.

In operation, crucible 48 is partly filled with powdered silicon and theapparatus is sealed. While hydrogen is admitted through the auxiliaryinlet 433 at an advantageous rate of flow of about oneliter per minute,the melt 29 is brought to temperature by the passage of a high frequencycurrent through theinduction coils. 42. A mercury .arc generator with a25 kilocycle per second frequency output has been found advantageous forheating. The melt is observed through the port M3 and the flow ofcurrent through the heating coils regulated so that the temperature ofthe melt, between 1450 C. and i550" C., is high enough. to prevent theformation of solid at the melt surface.

Upon melting, -the bulky powdered silicon is reduced considerably involume; This-volume reduction, notto partially or wholly liquid, thenadditional: powderedsilicon may be admitted stepwise from thereservoir-438- through the silica tube 435 to the melt 49. A silica tamp436' is used to mix the fresh charge with the molten material alreadypresent. Stepwise addition of small amounts of the powder is preferred,as such inhibits cak'ingof the freshmaterial and facilitatesliquefactionof the charge. Addition of fresh powder is usually stopped before thelevel of the melt is nearer than one inch to the top of the crucible 48;

After a full molten charge is obtained, hydrogen saturated withwatervapor at a convenienttemperature, such as 10 C., is then passed throughthe tube 437 and bubbled into the molten silicon at a rate ofabout' oneliter per minute. A slow stream ofpure hydrogen or mixed hydrogen-watervapor may also be continuously fed through the inlet 433. can thus bemaintained without the necessity of passing so much'gas through the-melt49-as to cause spattering.

This auxiliary gas also provides afactor of safety-in case the tube 437should become clogged.

The treatment is continued, in this embodiment of the invention, for aperiod of about four hours or longer, depending on the degree ofpurification desired. Purification of the melt is brought about both atthe liquid-gas interface at the melt surface, and within the bodyof themelt by the passage of the hydrogen-water vapor mixture therethrough.

Finally, upon conclusion of the process, the tubes 435 and 437 areremoved from the melt 49. Then-the platform 43 and coils 42 are raisedat a slow and steady rate, conveniently one-eighth inch'per minute,relative to the tube 41, by means comprising the motor and gear box 44.This permits solidification of the melt from the bottom upward, withthose impurities which are preferentially soluble in liquidsilicon beingconcentrated and eventually frozen out in those portions of the siliconingot last frozen. This is an example of normal freezing used to purifysilicon.

Fig. 5 is a detailed drawing of one modification of several deviceswhich have been used as endcaps similar to the one shown as upper cap112 in Fig. 1. In the figure, a tube 51, such as the quartz tube 11 ofFig. 1,.is fitted with a metal rim =52, conveniently of' brass. The rim52 is joined to the tube 51 with an adhesive'seal 52,- conveniently ofpicein wax. Therim 52 is fitted into a machined cavity 55 in a hollowmetal cylinder 54, also conveniently of brass. A hollow metal inlet oroutlet tube 56 is entrant into the cavity55, providing a free path intothe interior of tube 51 through the cylinder 54-.

A metal shaft 57 istightly fitted into the hollow cylinder 54 so thatwhen said shaft 57 is lubricatedwith a material such as stopcock greasean essentially gas-tight seal between the shaft 57 and cylinder 54 iscreated. The shaft 57 is threaded at its upper end 5%, and screws into arotatable metal fitting 58 secured to the cylinder 54 by a metalscrew-on cap 501. The cap 501 and a flanged'portion' 59 of the fitting58*inhibit motion of the fitting 58 other than rotary motion. Aprotruding metal stud 506 fastened to the middle portionof the shaft 57fits into a' slot 507 machined in-the side of the cylinder 54. Said'slot5'97 isonly wide enough to permit a fit of the stud 506- therein,preventing rotation of the-shaft 57 in the cylinder 54. The length ofthe slot 507 is such as to allowconsiderable vertical play, convenientlyabout one" and one-half inches-{of the shaft 57 within the--"cylind'er54. a 7

moderate volume decrease The powder is of low A favorable atmosphere Atits lower end 502, the shaft 57 is hollowed to accommodate a rod 503which may be, as suggested in Fig. 1, a support rod of silica. Byvertical movement of the rod 503 in the hollowed portion 502 of shaft57, the length of rod 503 extending into the tube 51 can be roughlyadjusted. The rod 503 may then be secured in position by tightening achuck 504.

By use of the device of Fig. or another similar device, the verticalmovement of the support rod 503 in the tube 51 may be finely controlled.Rotary motion imparted to the threaded fitting 58 is translated intovertical reciprocal movement of the shaft 57 because of the restraint toshaft rotation imposed by the slot 507 and stud 506 fitting therein. Theslot 507 permits only vertical movement of the stud 506 and shaft 57.Said vertical reciprocation of the shaft 57 causes an identical motionin the support rod 503 held rigidly thereto by the chuck 504.

When used in apparatus such as that shown in Fig. 1, the support rod inturn is cemented to a silicon bar being refined. Upon formation of amolten zone in the silicon rod, the volume decrease in the siliconcaused by melting, which volume change results in a reduced diameter ofthe silicon bar in its liquid portion, can be compensated by ashortening of the total length of the silicon bar using the apparatus ofFig. 5. Also, before solidification of the last zone, volumediscrepancies in the silicon bar expected from expansion uponsolidification can again be compensated by an appropriate lengthadjustment.

Such volume changes upon change of state and the problems they pose inother methods of zone refining are described in the paper by W. G;Pfann, entitled Change in Ingot Shape During Zone Melting, published inthe Journal of Metals, volume 5, November 1953, at pages 1441 and 1442.

In Fig. 6, the resistance characteristics of a silicon .rod purifiedusing the apparatus of Fig. 1 are shown graphically. The curves show theresistivity of the silicon rod measured in ohm-centimeters and plottedon the ordinate on a logarithmic scale. The units of the abscissa areinches, and the variable plotted is the distance from the end of the rodat which a molten zone is started through the rod. In curve 61 is shownthe resistivity of a silicon rod containing boron impurity introducedinto the material upon its formation by mixing boron trichloride withsilicon tetrachloride and reducing both compounds with hydrogen. Onezone-melting pass has been made in the rod prior to the measurements. Bypassing a molten zone through the length of the rod, the concentrationof the impurity in the rod can be rendered fairly uniform, as indicatedby the long flat portion of the curve 61. The zone pass, or movement ofthe silicon rod through the induction coil as shown in Fig. 1, was made,for this sample, at a rate of 0.1 inch per minute. During the pass, dryhydrogen was conducted through the chamber containing the silicon beingtreated.

After an initial pass, a second pass was made, this time as hydrogensaturated with water vapor was flushed through the apparatus. After thezone had traversed five inches along the total rod length, the source ofwater vapor was disconnected from the hydrogen line and once more dryhydrogen only was used to flush the silicon sample. The resultinginfluences of this treatment on silicon resistivity are shown in curve62. A substantial increase in resistivity, corresponding to a decreasein the concentration of impurities has been brought about. Also, at thatpoint at which water vapor was excluded from the hydrogen atmosphere, :1decrease in resistivity, signifying a lowered efiiciency in impurityremoval, is apparent.

Prior to the measurements which were used to plot curve 63, the samesilicon sample used in the previously mentioned experiments was oncemore zone refined in an atmosphere exclusively of hydrogen, no watervapor being present. No significant improvement in the resistivity valueis noticed in curve 63 above that value of the resistivity obtained bythe wet hydrogen treatment preceding the measurements of curve 62.Clearly, the presence of water vapor in the ambient atmosphere isbeneficial in obtaining a significantly higher degree of purity in thematerial under treatment.

Curve 64 shows the effect on the silicon resistivity of a secondzone-melting treatment using hydrogen saturated at 0 C. with watervapor. Roughly a three-fold increase in resistivity has been broughtabout by this second water-vapor purification. The water vapor was keptas a component of the atmosphere throughout the entire zone-melting passin this example.

Finally, in curve 65, changes in the rate of zone travel through thesample were explored to find their effect on the purification, wethydrogen again being used as the atmosphere. The flat, high-valuedinitial portion of the resistivity curve resulted. from passing thefirst two inches of the silicon rod through the plane of the inductioncoil at a rate one-half that used in the other experiments, that is at0.05 inch per minute rather than at 0.1 inch per minute. At the sharpbreak in curve 65, the faster rate was again used, resulting indecreased purification and consequently decreased resistivity. The sharpupturn of the resistivity curve for the end of the rod at which the lastzonm were refined was caused by maintaining one zone, which was fixed bythe dimensions of the induction heating coil at 0.25 inch in length,molten for a period of four minutes. The increased time interval duringwhich the molten zone was kept in contact with wet hydrogen thus appearsresponsible for the increased degree of purification and higherresistivity values ob served in the initial and final segments of curve65 as compared with the central portion in which the usual rate of zonetravel was maintained.

In Fig. 7 also, the resistance characteristics of a silicon rod purifiedusing the apparatus of Fig. 1 are shown graphically. The curves show theresistivity of a silicon rod measured in ohm-centimeters and plotted onthe ordinate on a logarithmic scale. The units of the abscissa areinches and the variable plotted is the distance from the end of the rodat which a molten zone is started through the rod.

Curve 71 is a plot of the resistivity of a boron-doped rod subjected toan initial zone pass made in dry hydrogen at a zone travel rate of 0.1inch per minute. The

pass, similar to that made before taking the data shown in curve 61 ofFig. 6, was made to level the impurity concentration'in the rod prior towater vapor treatment. Another zone was then started through the rod 2inches from the rod end and allowed to move at a rate of 0.05

inch per minute to a position 4 inches from the end of the rod.The-subsequently measured resistivity of this portion of the rod isindicated by curve segment 72. Water vapor was then mixed with thehydrogen atmosphere surrounding the rod. The previously dry hydrogen wassaturated at minus 18 C. to give a partial water vapor pressure of0.94-millin1eter in the gas mixture. The resultant increase in siliconresistivity, corresponding to a decrease of boron concentration, isshown by the portion 73 of the resistivity curve.

When 7 inches of the rod had been processed, the movement of the moltenzone through the rod was stopped for about five minutes While theapparatus was modified to permit saturation of the hydrogen carrier gaswith water vapor at a temperature of 0 C. During this time also the newatmosphere, containing a higher vapor pressure of water, was used toflush the apparatus of its .previous gaseous contents. This hiatus inzone movement during equilibration, with one molten zone segment beingexposed to water vapor for a relatively extended period, led to anincreased, time-induced, purification indicated by a sharp peak 76 inthe resistivity curve. After equilibrium with the new atmosphere had fEl 1' been reached; movement ofl -the-:zone at 0.05: I inchper minutewas reinitiat ed With a= newhydrogen atmosphere containingwaterr vaporat a partial pressure'of 4.6 millimeters. Purification brought about bywater-vapor at this: concentration is shown by segment 74iof= the-plotin Fig. 7.

When roughly 9inches of'theirod had-been processed, .-zone- -movementwas again: stoppedandthe apparatus modified to permit saturation of thehydrogen carrier-gas at'll" C. Another peak 77 resulted-from'theincreasedtime of exposureof a single molten zone to a purifyingatmosphere. 'After abOutfive minutes, zone movement --was-again--begunat a rate of 0.05 inchperminuter The molten zone travellingtothe end 'ofthe rod-was in con- -tact with hydrogen saturated with waterwapor; atll"C.

The vapor was presentat a partial pressure of 9.8 millimeters. 'I Thispart of the process is indicated by segment 75 of the curve.

The concentration of boron in the-silicon can be com- ";puted -foragiven value of the silicon resistivity." Such-- calculations were madefor the resistivities measured on the-plateaus 72 73, 74'and '75 of theplot; "The concen *trations so-oalcula'ted were expressed as :a-ratio )o"where -(B) "is" the concentration 'of boron foundb'efore water-vaporpurification, asindicatedby the-resistivity mechanism involving anheterogeneous reaction at the ---value denoted 72-,---and (B) is theconcentration of boron the silicon after treatment with=an atmospherecon tainingaspecified partialpressureof watervapor, as in- -'-'dicated*by the resistivity'value-measured at573, .74. and -75 on the plot.

A plot of log M against the squarezrootof the respective vapor pressurevalues-of water. used in -treating the silicon priorltomakiingttheresistivity measurements is linear.

flFrom consideration of'the data of Figs. 61and 7,-and :other similarplots, direct dependence of the value ofthe logarithmic purificationratio,

- .on time and-on the. square root of the water. vapor pressurevaluecan: bepostulated.

"It may further-be conjectured that theboronremoval mechanism; includesa heterogeneous reaction-,at the molten silicon surface. In consequence,{the changes in 1concentration: observed during purification. should-bei -directly'proportional to themagnitude of the silicon area ,expiosed,to-the purifyingwatmosphere, and inversely proportional to;the' volumeof'v silicon being purified.

.An :ernpirical expression for the; purification process 1 hasbeenderivedfrom the observations given above. .The

-;;equation relatesthe pertinent Variables in a, manner which 1. fairly.well: describes the purification mathematically:

log.

where :(B)'=boron coneentration'afterpurification (B) =initial boronconcentration before-purification K =proportionality 4 constant Asurface area of liquefied silicon exposed *to purifying The. aboveproportionality, derived .from T andjsubstan- :I ,tiated .by.experiment; appears. consistent with theview that thepurification iskinetically dependent on a removal rmelt surface.

- Although the kinetics of the removal may be reasonnably explained bythe above discussion, the equation "derived gives little insight intothe chemicalreactions which occur in the process. The presence oftraces" of borates in deposits of silicon dioxide found on the innersurfaces of the quartz tube jacket in the apparatus of Fig. 1- seems,however, to confirm the hypothesis that an oxidation of boron by watervapor is occurring, and that the boron is removed from the silicon byevaporation as an oxidized species.

The eificacy of the purification treatment is linked with thevolatility, or ease of removal, of the oxidized impurifrom the moltenmatrix. The ease with which such oxidized impurities escape isapparently dependent on their nature and the identity of the matrixmaterial as well as the temperature at which the impurities are to boil'oflf. Thus, the ease with which boron contaminants are removedfromsilicon by oxidation with watervapor indicates a high volatility forthe oxidation product at "temperatures between the silicon meltingpoint, approximately 1420" C., and about 1550 C. By escape of theimpurity from the reaction zone, an essentially irrevers iblepurification reaction is obtained in the case ofboron removal.

With aluminum, the evaporation of oxidation products is less favorable,and an equilibrium between oxidized and ,unoxi'dized species is set upin the molten silicon. Some oxidationapparently occurs,'but a majorityof the'original aluminumicontaminantremains unaifected. Nonoticeable-'efiectofthe Water vaportreatment on phosphorus inSilk/01111188 beemobserved.

"The water vapor treatment, then, is most effective in removing boronimpurities, and showseifectiveness in a partially" removing "aluminum"impurities. By coupling water vapor 'treatmentwith a zone refining step,alumi- "num,phosphorus;'andboron'may bee'liminated' as con-,

-taminantsfi"Phosphorus'impurities, largely unafiected by water vaportreatment alone, can also be 'remove'ctby "iiquefyingsilicon in-vacuum,as these' impurities are comparatively volatile boil from "the liquidmetal in vacuum.

The observations "mentioned above aredirectlyapplicable to siliconsemiconductor materials. Substantialdifierences'existbetweentheprocesses described and the "process forpurifying germanium taught in the copending rapplicatioirof If ,Hr'Scafiand H. C. Theuerer, Serial No.

' 23 6,662; earliermentioned.

Since germaniummay be liquefied in crucibles" which are'comppsedbf'chemically reactive materialsjsuch as graphite which mayact as areducing agent; 'the' possibility is' presented ofcontr'olling impuritylevehwithin'the germanium Inelt"by"bal'ancing the respective extent of"simultaneous oxidation and reduction reactions. Thus, for'example,-aluminum" impurities may be substantially completely oxidizedin germanium byxexp'osureof the *germanium'meltto water vapor." If watervapor isthen excluded from theatmosphere' over the melt, a reduction"of-the oxidized aluminum to the metallic impurity is free to "occur iffthernelt is "kept in contact with'aneducing '"agentsuchas the' graphitecrucibles usuallyused to contain the melt.

In the silcon'refining process, 'the 'use" of graphite crucibles is "notpossible as; sorptionof 'the'm'oltensiiicon into the 'gr aphite occurs.Non-reactive," nomreducing, non-contaminating-containers; such as thosemade of'silica, are used, or'th'e silicon is refinedin *apparatus,such'as that schematized: inFig. -l,' 'which 'doesnotrely'ion'containmentof-the siliconin'any vessel.

In silicon, further, aluminumyfthe'jimpurityfeasily controlled ingermanium, 'is"'only "partially oxidized by water vapor treatment; asearlier"'me'ntioned.- An equilibrium systemisforniedlwhen approxiniately"16' percent "of the llimifll llh iS oXldiZe'd. Complete relfioli'albyoXidapressures.

13 tion is not encountered. Also, for the boron removal process, whichproceeds very efficiently in silicon refining, the efliciency may be inlarge part dependent on the escape from the melt of boron impurities asoxidation products. This escape of boron from the reaction zoneprecludes the possibility of impurity concentration control by opposingreduction reactions as is used in re fining germanium.

Returning to consideration of the details of water vapor refinining, asnoted before, the removal of boron impurities from silicon is dependenton the square root of the partial pressure of water vapor in contactwith the silicon. Themelt may be treated with water vapor in a partialvacuum, or may be conveniently treated with a moist neutral gas atatmospheric pressures or higher Where it is desired also to removerelatively involatile phosphorus compounds, vacuum treatment may beindicated. The lowering of pressure over the melt Ihas been foundelfective in aiding phosphorus evaporation. Normally, however, the zonerefining and normal freezing processes illustrated by Figs. 1, 3, and 4are done while a neutral gas, preferably hydrogen, containing watervapor surrounds the silicon being refined.

By a neutral gas is meant one for Which no undesirable side reactions ofinterfering magnitude occur. The purification process functions moreefiiciently when hydrogen is used as a carrier for water vapor than whennitrogen or the rare gases are used. Such a difference indicates thatthe system is not completely indifferent to the gas used, and that,probably, the gas is not inert in the sense of not, in some way,influencing the process.

The neutral gases herein mentioned are intended to be distinguished fromgases such as oxygen, chlorine, phosphine or carbon dioxide, which, byextensively oxidizing or contaminating or otherwise actively andcompetitively reacting with the silicon being purified, might nullifythe benefits derived from the water vapor treatment.

While the partial pressure of water vapor in such mixtures with aneutral gas, or even the pressure of water vapor in partial vacuum, maybe raised to fairly high values, it has been found convenient to keepthe pressures below the equilibrium partial pressures of water vaporover water at room temperatures. Use of pressures higher than about 25millimeters, which is the vapor pressure of water at 25 C. to 26 C.,requires keeping all portions of the gas system above room temperatureto avoid condensation. By saturating the gas mixture with water attemperatures below room temperature, no precautions against condensationin lines at room temperature need be taken. Similar considerations applyfor partially evacuated systems containing some water vapor. The vaporin the system is best equilibrated with water at a temperature lowerthan the temperature at any other point in the system. As mentioned,equilibration at temperatures below room temperature is most convenient.

Thus, a water vapor pressure of 4.5 millimeters of mercury isadvantageously used in many application of the purification technique.This pressure is the vapor pressure of water at C. In practice, theequilibration is best accomplished by saturating or partially saturatingthe dry neutral gas used, preferably hydrogen, at room temperature. Themoist gas is then passed through a trap at the desired equilibrationtemperature, conveniently 0 C., with any excess vapors being condensedin the trap. This procedure of first loading a gas with water vapor at ahigher temperature, then condensing excess vapor by chilling at thedesired equilibrium temperature, assures that the gas will be fullysaturated at the equilibrium temperature. Attempting to saturate at theequilibrium point itself may require additional precautions to assurethat true equilibrium has been reached and that the gas is, in fact,completely saturated.

Though a convenient upper limit on the saturation temperature might beset at room temperature, then, the

14 most advantageous operating range lies between 0 C. and 11 C. Above11 C., at which temperatures the equilibrium vapor pressure of water isgreater than 9.8 millimeters, the oxidation of silicon occurringsimultaneously with the purification reactions may noticeably interferein the process. The molten silicon surface may become too coated withsilicon oxidation products to permit efiicient oxidation and removal ofunwanted boron. At the low temperature end of the scale, purificationhas been observed even where equilibration of the moisture content hasbeen made at --18 C., giving a water vapor partial pressure of only 0.94millimeter. Even lower temperatures and pressures can be used if theyafford practical advantage. A balance of convenience and a desire forfairly rapid purification suggest that 0 C. is generally the besttemperature for saturation.

If a carrier gas is used in the purification, rather than a water vaporstream in partial vacuum, hydrogen is advantageously employed. Helium orargon are also suitable alternatives, though the use of a high frequencycurrent in the induction coil may give rise to interfering glowdischarge phenomena. Nitrogen may be used at high frequencies withoutthe interferences observed for the rare gases. As noted earlier, thepurification process appears to be less efficient with these gases,however, than when hydrogen is used as the water vapor carrier.

When hydrogen is used, it may be desirable to remove traces of oxygenfrom the tank gas. Such removal can be accomplished by passing thehydrogen over palladinized alumina, for example. Condensible impuritiesin the neutral gases are removed by passage of the stream through aliquid nitrogen trap. Adsorbent charcoal in the trap helps in removingcontaminating impurities.

A typical purification and saturation train, in the preferred case usinghydrogen, will involve successive passage of the gas stream, then, overa supported catalyst to convert oxygen impurities to water, through acharcoal trap at liquid nitrogen temperatures to condense or adsorb theremaining gaseous contaminants, through a water bubbler conveniently atroom temperature 'Where saturation or near saturation is accomplished,and finally through a condensing trap conveniently at 0 C. where excesswater vapor is removed from the gas stream. The stream, containing apartial pressure of water of 4.6

millimeters, is then fed to the apparatus holding the moltensilicon.

The rate at which the gas is passed over the molten, silicon surface ischosen to conform with the rapidity of purification desired by theoperator. The partial vapor pressure of water in the atmosphere is ofprimary importance in affecting the rapidity of purification, and therate of flow of the gas stream is adjusted at a reasonable value toconform with the water vapor content decided upon. Convenience inhandling the gas may be a factor determining flow rate. The magnitude ofthe surface area of the silicon exposed to the purifying atmosphere andthe over-all dimensions of the apparatus in which purification isoccurring may also be of importance. No one of the variables hereconsidered is, taken singly, critically determinative of the gas flowrate. One skilled in the art can easily balance the influential factorsto accommodate them to his specific practise of the invention.

An extremely slow rate of flow for the purifying atmosphere, or anessentially static atmosphere, may result in localized depletion ofwater vapor from the gas volume in contact with the molten silicon. Sucha decrease in the effective partial pressure of water vapor in thereaction zone would, by essentially removing a requisite reactant,decrease the rate of purification obtainable. On the other hand, anexcessively rapid gas flow, while not harmful, would be wasteful andunnecessary.

In the floating zone apparatus depicted in Fig. 1, a rate of flow ofmoist hydrogen of 1 liter per minute has f ed in afoot-size 15 fbeientfoitimdjls'atisfactory. This rate is also advantageously "the" cruciblezone'melting technique shown in 'l? ig. 3. 'fFolrthe normal freezingcrucibleprocess used ""with the 'apparatus shown in Fig. 4, flow ratesof about "b'bling through the silicon melt and for the auxiliary "j gasflow admitted to the silicon'surface through a separate fta'lihedori'the'e scaping gas to prevent air from leaking the system andexcessively oxidizing the moltensilieing purified. I Th'e'ti'me duringwhich the purifying atmosphere and ""the'melt "are kept in contact isgoverned by the extent "to'which purification is desired; Other factorsbeing f equal; the rel'ation'j'given earlier shows that the logarithm"ffof th e"r atio of boronconcentrations after and before puriific tionis'dire'ctly proportional to' the time during which'icontactis'rnaintained. When the water "vapor fpro'cessispoupled with otherrefining steps, the time required for these other'steps may be a factorto be consid ered. As the extent of 'thepu'rification accomplished byZone refining is linked withthe number of zone passes made,'fconcurrentzone refining and water vapor treatmentm'ay be continued convenientlythrough whatever period of time is 'required'for the zone refining step.

f If boron need notbe'exte'nsively removed, a dry rather.

an a'moist neutral gas may be used duringsome portion' of the'zonerefining technique.

'Generally'speaking, eventhe shortest contact between "f'themelt' andwater vapor will result in boron removal.

By permitting the contact to continue for a period of "hours, ashighadegree of boron removal as is desired may beobtained. The rate of flowof moist gas over "the melt may alsobe adjusted with a view to the totalamount of'time during which treatment is to be con "tinued.

" of 0.03 ohm-centimeter.

In the'apparatus shown in Fig. 4, the passage of hydrogen saturated at22 C. with water vapor through a 'sili'conmelt for'three' and one-halfhours has been ob- "served to convert p-t'ype silicon, of an originalresistivity of 0.05 ohm-centimeter, to n-type silicon with a resistivityAfter a subsequent removal of phosphorus by vacuum zone refining, p-typesilicon of 2.5 ohm-centimeter resistivity'has been obtained. Such"changes, 'where'the water vapor treatment alone is largely responsiblefor boron removal, can be determined to correspond to a reduction of theoriginal boron con- "centration'from'approximately M boron atoms percubicfc'entimeter to a value of 6.5 10 boron atoms per cubic centimeter.A ,three" and one-half hour treat- 1nent" has thus removed 99.35 percentof the original boron contaminant.

Finally, in the use of water vapor refining, the silicon "melt ispreferably kept at temperatures between 1420 C. andl550 C. Temperatureshigherthan 1550 C. inlay, however, be reached locally in the interiorportions of melt or ingotbeing refined. Generally the figure of i550" C.is' often a practical upper limit on the temperatures at which zonerefining or normal freezing processes can be run. Silica apparatus,commonly used when theset echni'ciues are practised'on silicon, tends tosoften'at higher'temperatures. The water vapor process, fused alone'o'rwith simultaneous zone-melting or normal freezing, may'nms be alsolimited to temperatures below 1550 C. if apparatus made of silica isemployed.

Itjis tobe noted that the method of purification herein "destined hasbeen found fieetive iQ t e removal of iml iter'fperj minute of moist gasareused both for the "coupled with water vapor treatment.

centration of 1.9( 10 atoms per cubic centimeter. The

rod was subjected to two zone passes at a travel rate of 0.05 inch perminute, in apparatus similar to that of Fig. 1, while an atmosphere ofhydrogen saturated with water vapor at 0 C. surrounded the rod. Thistreatment was followed by nineteen zone passes in dry hydrogen: ten

at a travel rate of 0.2 inch per minute and nine at a travel rate of 0.1inch per minute. The resulting silicon, from which boron had beenlargely removed by the water'vapor treatment and phosphorus by thesubsequent Zone passes in dry hydrogen,was p type, with a resistivity of3000 ohm-centimeters. The boron and phosphorus concentrations in therefined material were detertriined by'lowtemperature Hall eifectmeasurements to be about 4.3(10 atoms per cubic centimeter and 3(10atoms per cubic centimeter, respectively. This isthe purest siliconknown to the inventor to have been so far produced. It should beunderstood that, as changes and variations may be made in the inventiondescribed above without departing from the scope and spirit of theinvention, the embodiments and examples of the invention given hereinare illustrative only and should not be construed as being limiting inany manner.

What is claimed is: v

1. The method of refining silicon which comprises establishing a moltenzone in a mounted silicon body, the size of said zone being such thatsurface tension forces in the zone keep the silicon body integral, andadvancing said molten zone through said silicon body while producing afiow of a neutral atmosphere comprising water vapor having a partialpressure of from about 0.94 millimeter of mercury to about 25millimeters of mercury over the surface of the said molten zone so as toproduce intimate contact therewith. v

2. The method as described in claim 1 for which the partial pressure ofwater vapor in said neutral "atmosphere is about 5 millimeters ofmercury.

3. 'The method as described in "claim 1 in which said neutral atmosphereconsists of a mixture of hydrogen and water vapor, the partial pressureof said water vapor being about 5 millimeters ofmercury.

4. The method of refining silicon which comprises liquefying a siliconmass, maintaining said mass at a temperature between 1420 C. and 1550C., passing a gaseous mixture of hydrogen and water vapor through saidmolten silicon till the silicon has been purified tothe extent desired,said Water vapor having a partial pressure of from about 0.94 millimeterof mercuiy to about 25 millimeters of mercury and then directionallysolidifying-the molten silicon by a slow progression of a solid-liquidinterface, first formed at one end of the mass, throughout theremainingmelt.

5. The process as described in claim 4 for which said mixture ofhydrogen and water vapor contains water vapor at a partial pressure ofabout 5 millimeters of mercury.

6. The method of refining silicon which comprises establishing at leastone molten zone in a siliconbody, ad-

'the surface oftlhe said molten zone so as to produce intimate contacttherewith.

17 18 7. The method of claim 6 in which the said atmosphere comprisingwater vapor having a partial ressure of from is partially evacuated.about 0.94 millimeter of mercury to about 25 millimeters 8. The methodof claim 6 in which the said atmosof mercuryh I t'aJl i p e e conslstsessen1 y of said water vapor together References Cited in the file ofthis patent with at last one neutral gas. 5

9. The method of claim 8 in which the said neutral UNITED STATES A Sgas'is hydrogen. 2,402,582 Scafi June 25, 1946 10. The method ofrefining silicon which comprises 2,631,356 Sparks etal Mar. 17, 1953bubbling a gas through a mass of molten silicon, said gas 2,739,088Pfann Mar. 20, 1956

1. THE METHOD OF REFINING SILICON WHICH COMPRISES ESTABLISHING A MOLTENZONE IN A MOUNTED SILICON BODY, THE SIZE OF SAID ZONE BEING SUCH THATSURFACE TENSION FORCES IN THE ZONE KEEP THE SILICON BODY INTEGRAL, ANDADVANCING SAID MOLTEN ZONE THROUGH SAID SILICON BODY WHILE PRODUCING AFLOW OF A NEUTRAL ATMOSPHERE COMPRISING WATER VAPOR HAVING A PARTICALPRESSURE OF FROM ABOUT 0.94 MILLIMETER OF MERCURY TO ABOUT 25MILLIMETERS OF MERCURY OVER THE SURFACE OF THE SAID MOLTEN ZONE SO AS TOPRODUCE INTIMATE CONTACT THEREWITH.
 10. THE METHOD OF REFINING SILICONWHICH COMPRISES BUBBLING A GAS THROUGH A MASS OF MOLTEN SILICON, SAIDGAS COMPRISING WATER VAPOR HAVING A PARTIAL PRESSURE OF FROM ABOUT 0.94MILLIMETER OF MERCURY TO ABOUT 25 MILLIMETERS OF MERCURY.